HAMMER update 24-Apr-2008

From:

Matthew Dillon <dillon@xxxxxxxxxxxxxxxxxxxx>

Date:

Thu, 24 Apr 2008 14:57:21 -0700 (PDT)

HAMMER is going to be a little unstable as I commit the crash
recovery code. I'm about half way through it. Meta-data updates
to the disk media have now been separated out. I have a few things
left to do before crash recovery will actually work:
* I have to flush the undo buffers out before the meta-data buffers
* Then I have to flush the volume header so mount can see the updated
undo info.
* Then I have to flush out the meta-data buffers that the UNDO
info refers to.
* And, finally, the mount code must scan the UNDO buffers and perform
any required UNDOs.
The idea being that if a crash occurs at any point in the above
sequence, HAMMER will be able to run the UNDOs to undo any partially
written meta-data. HAMMER would be able to do this at mount-time and
it would probably take less then a second, so basically this gives us
our instant crash-recovery feature.
One interesting outcome of the separation work I just committed is
that the frontend VOPs are *massively* disconnected from backend disk
I/O now. In coming weeks I hope to take advantage of this separation
to remove the remaining stalls and significantly improve HAMMER's
performance.
-Matt